The present invention relates to a magnetic resonance imaging apparatus and a magnetic field forming apparatus, and more particularly to a magnetic resonance imaging apparatus which generates an image of a subject on the basis of magnetic resonance signals from the subject in an accommodating space in which a magnetostatic field is formed, and a magnetic field forming apparatus which forms a gradient magnetic field in the accommodating space wherein the subject is accommodated and the magnetostatic field is formed.
A magnetic resonance imaging (MRI) apparatus is known as an apparatus which can pick up sectional images of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon. Magnetic resonance imaging apparatuses are used in many different fields including medical applications and industrial applications.
When sectional images of a subject are to be picked up by using a magnetic resonance imaging apparatus, first the subject is placed in an accommodating space in which a magnetostatic field is formed, and the spinning direction of protons in the subject are aligned in the direction of the magnetostatic field to achieve a state in which a magnetization vector is obtained. After that, a nuclear magnetic resonance phenomenon is generated by irradiation with an electromagnetic wave of the resonance frequency, and the magnetization vector of the protons is thereby varied. And the magnetic resonance imaging apparatus receives magnetic resonance signals from the protons returning to their original magnetization vector, and generates a sectional image of the subject on the basis of the received magnetic resonance signals.
For such magnetic resonance imaging apparatuses, pulse sequences for switching the gradient magnetic field at high speed, such as the high speed EPI method, have been developed to meet the need for reducing the image pickup time. In forming a gradient magnetic field by high speed switching, mechanical distortions arise in the gradient coil to invite vibration as an electromagnetic force works on the gradient coil at the time of rising or of inverting. This vibration may give rise to noise or deteriorate the image quality.
For this reason, vibration attributable to the gradient coil in magnetic resonance imaging apparatuses is suppressed by using a dynamic damper (cf. for instance Patent Document 1 and Patent Document 2).
[Patent Document 1] JP-A No. Hei 3-32643
[Patent Document 2] JP-A No. Hei 6-90919
Here, the dynamic damper is so arranged, in an accommodating space in which a subject and a gradient coil are accommodated, as to be held between the gradient coil and a magnetostatic field magnet.
For this reason, no sufficient space can be secured for installing the dynamic damper, and accordingly the mass of the weight of the dynamic damper cannot be increased, sometimes making it impossible to adequately suppress the vibration attributable to the gradient coil. Further, in order to complexly prevent the vibration of behavior in a plurality of natural vibration modes, it is necessary to perform adjustment of the supporting rigidity of the dynamic damper in each spatial direction, but as this adjustment is difficult, it is sometimes impossible to sufficiently suppress the vibration attributable to the gradient coil. Especially when the subject is to be scanned in a pulse sequence in which the gradient magnetic field is switched at high speed, vibration attributable to the gradient coil might actualize such troubles as the occurrence of noise or the deterioration of the image quality.